1. Related Applications
This invention is an improvement of application Ser. No. 408,542 filed Aug. 16, 1982 by Samuel C. Kingston for a "Dual Channel Clock Recovery Circuit".
2. Field of the Invention
This invention relates to the detection of covert communication signals. More particularly, this invention relates to an improved novel circuit for detecting the presence of signals which typically are received at very low signal to interference and noise ratios.
3. Description of the Prior Art
The general class of signals to which the present invention is directed are commonly referred to as periodically keyed random modulated signals. For example, communications intelligence is often transmitted in coded form. One former way of denying data access to the enemy is to transmit the data stream in direct sequence spread spectrum format. It is extremely difficult to detect data signals embedded or encoded in such spread spectrum format because the signal-to-noise ratio is so low as to make detection difficult.
Before it is possible to attempt to decode direct sequence spread spectrum coded data signals, it is necessary to determine that such coded signals are actually being transmitted. This invention is directed to the problem of detecting that such coded signals are being transmitted and is not directed to the problem of decoding such signals.
It has been suggested that radiometers or power signal detection devices may be employed to determine if periodically keyed random modulated signals are being transmitted. When such signals are received at a receiver, it is often impossible to distinguish them from the received noise, thermal background noise, other transmitted signals and interferring emission signals. It is possible that the power level of the signals which are to be detected, do not exceed the background and interference signals mentioned above thus, it is often impossible to employ radiometers and power detection devices to detect the presence of low power periodically keyed random modulated signals.
When a radiometer is employed to detect the presence of a signal, then the threshold of a detector must be set very close to the signal levels. Changes in either the interference levels or the threshold levels will affect the sensitivity of the receiver which results in false alarms or reduced sensitivity. For example, if the threshold of a radiometer is set to detect the desired signal at minus 20db signal-to-noise ratio, then a one percent increase in interference level will cause a false alarm.
It has been suggested that since periodically keyed random modulated signals by definition change symbols at a fixed rate, it may be possible to detect a periodic repetition as a clock signal even though the data signal is not discernable.
In the above-mentioned application Ser. No. 408,542, the received periodically keyed random modulated signals were split into two separate and distinct channels, each comprising a narrow band pass filter. One of the narrow band pass filters in one of the channels was tunned to a frequency higher than the expected center frequency of the periodically keyed random modulated signals and the other narrow band pass filter was tunned to a frequency lower than the expected center intermediate frequency. The distance between centers of the narrow band pass filter frequencies was designed to be approximately the clock frequency to be detected, thus, both channels contain spectral signals indicative of the clock signal to be detected. The outputs from the narrow band pass filters were connected to a mixer to provide an output signal therefrom representative of a clock having a frequency equal to the periodically keyed random modulated signal. The output from the mixer was detected and/or analyzed for the presence of a clock signal by comparing the mixer output with a reference frequency or a reference voltage. While this apparatus and structure is extremely effective in detecting the presence or absence of periodically keyed random modulated signals, the signals being analyzed and/or compared include interference lines representative of continuous wave (C.W.) interference signals.
It would be extremely desirable to eliminate the C.W. interference signals that appear the same as clock signals and can produce false alarms or false indications of such clock signals.